fn main() {
    let name = "张山峰";
    let age:u32 = 20;
    println!("name is:{}", name);
    println!("age is {}", age);
    let a = 12;
    println!("a is {}", a);
    let a = "abc";
    println!("a is {}", a);
    // let并不是变量定义，而是一个解构
    let mut str = String::from("hello");
    println!("str is {}", str);
    str.push('1');
    let ll = Some(123);
    match ll {
        Some(1) => { println!("ll is 1") }
        Some(num) => { println!("ll is not 1 but {}", num) }
        _ => println!("ll is unknown")
    }
}
